John@John-The-Ripper:~$ usermod -l John serg
usermod: user 'serg' does not exist
John@John-The-Ripper:~$ adduser serg
adduser: Only root may add a user or group to the system.
ater.

Then I tried to go to the GRUB menu and from there log in as a root. I did this, but however when I tried to create user serg, it gave me an error that group already exist. When I tried to change /etc/passwd it said permission denied.